A. Field of the Invention
The present invention relates to a bed-of-pins test fixture and, more particularly, to an in-circuit test fixture for use in making a non-functional electrical inspection of individual components and/or circuit paths on a printed circuit board or wired backpanel assembly.
B. Description of the Prior Art
The present invention is concerned with the testing of printed circuit (PC) boards, primarily the testing of PC boards before and after the individual components have been mated thereonto and connected.
There are basically only two types of PC board testers, namely in-circuit and functional. All early automatic test: systems were-functional testers, i.e. connections were made only to the input and output terminals of the board and signals were conducted to the input terminals which simulates the input signals the PC board would experience in an assembled unit. The output terminals were monitored to determine whether or not the entire board was functioning properly.
While this approach was and still is very effective in separating good and defective PC boards, it is ineffective in isolating the cause of the test failure of defective boards. As a result, troubleshooting at a manual station is still required.
To overcome this shortcoming of functional testers, a totally different approach to PC board testing was developed, what is herein referred to as an in-circuit tester. The in-circuit tester does not approach testing on an input/output basis. Rather, testing is achieved by making contact with each electrical point on a circuit board and sequentially monitoring each and every circuit component and each and every circuit path. In this manner, solder shorts, opens, missing parts, wrong parts, backwards or improperly seated parts, and out-of-tolerance parts may be isolated and identified.
A complete in-circuit test system includes a simple sequencer-type controller which switches from component to component on a PC board while monitoring that component's performance. Programming can-be done very simply using microprocessors and/or minicomputers.
The most critical part of an in- circuit test system is the test fixture which must be configured for each part to be tested. The test fixture has the responsibility of making a reliable electrical connection from the controller to every node on the circuit board. This is typically done by using a bed of pressure pins that contact the foil side of the circuit board. A number of schemes exist for keeping the board in contact with the pins during test. This can be done manually or through the use of a vacuum-actuated fixture, both of which are well known in the art and maintain and distribute a holddown force evenly over the board.
A typical bed-of-pins fixture uses a blank panel with holes drilled on the same matrix as the PC board to be tested. The hole sizes are large enough to accommodate a sleeve which has a square tail extruded on the end for wire wrapping. A spring-loaded contact pin is inserted into the sleeve for making contact with the PC board. The fixture also has holes drilled on the other end of the board and solid feedthrough-type pins are installed which make contact with a transition head. The transition head usually has contacts which are the same as those which mate with the test PC board. The centers of the contacts on the transition head and feedthrough contacts on the interface part of the fixture are the same so that they will mate. Each sleeve is individually connected by an electrical lead to a corresponding solid pin.
The bed-of-pins test fixture provides a reliable method of testing for electrical defects. It provides the user with a simple and highly productive way of testing thousands of circuits in less than a minute in some instances. The main disadvantage with this system is cost. In most cases, a bed-of-pins fixture is cost prohibitive due to the high cost of the spring-loaded contacts and necessary accessories.
This being the case, the most widely used method of testing PC boards requires an operator to review a wire list or artwork to determine how the circuit should be connected. The operator must then proceed to touch a set of probes to all common circuits to verify their connection. The operator must also verify that circuits that are uncommon are not connected. A simple battery and light with positive and negative leads can be used and a buzzer is sometimes used as an ohmmeter. The disadvantages of this approach are great. Error is almost surely introduced by the operator. The time required to test a single circuit board is lengthy and the results of such test will always be open for criticism.
Since all of the systems for testing circuit boards have some disadvantage, it is very often the case that circuit boards are not tested prior to delivery to the customer. The user merely tries to use the board and work around problems, if possible. If the problems cannot be overcome, the circuit board is simply discarded and replaced.
In summary, all of the methods discussed above have one thing in common, i.e. they are cost prohibitive. Most cannot be used unless the quantities of product are large enough to support the large amounts of money required for necessary tooling. On the other hand, those not requiring high tooling costs, slow down production and result in high labor costs. The current testing methods, while adequate at times, do not meet industries needs most of the time. Thus, it becomes obvious that other, more economical, test methods must be created.